Camshaft phasers for varying the valve timing of internal combustion engines are well known. A phaser typically comprises a rotor element attached to the end of a camshaft and variably displaceable rotationally within a stator element driven by the engine crankshaft. Phasers typically are actuated by a pressure duty cycle of oil derived from the engine's main oil supply and selectively directed to chambers within the phaser to alter the phase relationship between the rotor and stator, and hence between the camshaft and crankshaft.
A torque-imposed instability is known in the art that can cause the phase relationship to vary from nominal during a rotational cycle of the camshaft. In opening a valve, the valve follower leaves the base circle portion of the cam lobe and begins to climb the rising edge of the eccentric portion, creating a resistive torque on the camshaft. At some point, the resistive (negative) torque reaches a maximum, then declines to zero, and then becomes a positive torque in the opposite direction as the follower descends the falling edge of the eccentric portion and the valve closes. Because of mechanical and hydraulic lash in the system, the actual rotor positions with respect to the stator may be significantly different from the intended nominal positions during valve opening and valve closing. The difference between the maximum negative and maximum positive angular departures from nominal is known in the art as “phaser instability.”
A typical cam phaser in good working order exhibits a characteristic level of instability due to inherent mechanical and hydraulic lash in the system. The phaser behaves somewhat like a spring-damper, absorbing the shocks from the valvetrain events. Depending upon the oiling system conditions (pressure, temperature, aeration, etc.), the hydraulic system may become like a soft spring rather than a stiff spring. When the hydraulic system is soft, the rotor oscillates more and tends to drift from its desired position. Some amount of such phaser instability is expected and acceptable, but excessive instability is undesirable, being indicative of poor system performance, and may affect the effective cam timing of the engine.
What is needed is means for measuring the level of instability continually during engine operation, detecting when the level of instability changes, and causing the cam phaser and engine to take predetermined action when measured instability exceeds a predetermined threshold level.
It is a principal object of the invention to alarm instability changes and to cause the phaser to take predetermined action to reduce the level of phaser instability to minimize potential problems such as engine malfunction and emissions increase.